If you do a quick sweep of how you are spatially, the implicitly understood layout of embodied self, you get a sort of onion pattern of consecutive layers radiating out from a self-centre which is not exactly in physical space, but out of which physical space is abstracted. The layers are like distinct shells and there is a sense that there is an ontological difference which defines, determines and stabilises them. But if you wonder how you come to 'know' this you immediately see that it makes no sense, what could possibly ground such a knowledge? It is the other way around, then. Because you think this is stable, so that it can position you, you invent an idea of an ontological difference which thickens into a sort of grey and neutral and objective stuff lining the concentric spheres, like dirt in the imaginary onion. That's the closest you get to matter, just an invented thing to keep open the relations of containment. It's a marvelous invention, since it comes out of nowhere and appears to do so much. But there can't even be such an idea because there's nothing like it at all that you can have directly known, it is an idea or representation with no possible referent at all.